Potentially destructive earthquakes such as those whose magnitude on the Richter scale is at least about 5.0, can produce damage over a wide area around the the earthquake epicenter. Earthquakes produce P (for primus) waves which are propagated as alternating compression and rarefaction of material similar to sound, and S (for secundus) waves which propagate by moving the earth at a right angle to the direction of the wave in a manner similar to propagation of surface waves in water. The P-waves travel at a rate of about 5.6 km per second (about 3.5 miles/sec), while the S-waves travel at a velocity a little more than half that of the P-waves (about 1.8 miles/sec). The P-waves are less energetic than the S-waves. Larger earthquakes can cause damage in areas 60 or more miles from the epicenter. It would be desirable if most of the potential victims could be alerted to the imminent arrival of destructive shocks, so they could seek shelter as by moving away from windows to locations under door frames and the like. While it would be possible to use an instrument that detects an earthquake near the epicenter to immediately sound alarms at location many miles away to alert persons of impending shocks, such systems would require an expensive network. A relatively simple apparatus that could warn persons of impending large shock waves, and that was of relatively simple and inexpensive construction, would be of considerable value.
Although persons become immediately aware of destructive shock waves, it is often useful to provide an independent detector which can awake persons who will sleep through the shock, and to shut off gas and/or electricity to minimize the possibility of fires. One type of detector includes a pendulum surrounded by a ring. Shaking motion of the earth causes the pendulum to swing against the ring and close an electrical circuit. U.S. Pat. No. 4,359,722 describes a detector of this type. A problem encountered with such detectors is that it is difficult to adjust their sensitivity. Persons who first install such a detector may want it to sound an alarm whenever a moderate shock is encountered such as from an earthquake of a level slightly exceeding 4.0 on the Richter scale whose epicenter is within about 30 miles. However, if there are too many "false alarms," as may be caused by slamming of doors or running in the house, such persons may wish to reduce the sensitivity, which is difficult to accomplish economically with a pendulum detector. A simple and low cost earthquake detector whose sensitivity could be easily adjusted, would be of considerable value.